This invention relates to an instrument reference system and more particularly relates to a system for calibrating an instrument pointer driven by a motor against a mechanical stop without using feedback control. The invention will be described with reference to an automotive instrument by way of example only but it will be understood that the invention is not limited thereto.
Calibration of a motor, such as a stepper motor, used in an instrument or equipment of the type used for automotive instrumentation, is important for correct operation of the instrument. Typically, an electronic-controlled instrumentation may take the form of a mechanical dial comprising a pointer driven by a stepper motor. To ensure proper function, the pointer location must be predictable. As is known from existing systems, an inexpensive way of achieving this, without using any form of feedback, is to incorporate a mechanical stop as part of the mechanism and to locate the pointer relative to this stop at xe2x80x9cpower onxe2x80x9d.
One known system for calibration of a stepper motor in automotive instrumentation provides that, at initial power up, the motor is driven at least a few degrees against the stop to ensure that it has made contact with the stop. To avoid jitter (small vibrations of the motor shaft) at the stop the frequency of the rotating magnetic field is increased quickly beyond that at which the motor can synchronise its movement with the field.
The drawback of this system, however, is that the jitter is most evident if the motor is already against the stop when the referencing procedure commences. Another drawback with this technique is that it relies on the pointer remaining against its mechanical stop once it has reached it. In practice it has been found that the pointer has a tendency to xe2x80x98bouncexe2x80x99 up to several degrees off the stop, especially when the motor has been moving at high speed when it hits the stop. Once this has occurred, the motor can not re-synchronise with the high frequency rotating field and remains stationary away from the reference position.
One possible solution to the xe2x80x98bouncingxe2x80x99 problem is to drive the motor and hence the pointer at a sufficiently low speed. This, however, is not desirable as it can take considerable time where the field must rotate for sufficient time to guarantee that the motor has reached the stop. In automotive instrumentation, this delay is particularly unsuitable.
It is therefore desirable to provide an improved calibration system for instrumentation, without using feedback control.
It is further desirable to provide a system with reduced jitter of the driving motor.
It is also desirable to provide a system which calibrates quickly and which can be produced at an economical cost.
According to one aspect of the invention there is provided a method of calibrating an electronically controlled, electric motor driven instrument comprising the steps of:
a. connecting electrical power to the instrument,
b. driving the motor at full torque for a first predetermined radial distance whereby the motor or a pointer driven thereby contacts a stop,
c. driving the motor at a reduced torque for a second predetermined radial distance whereby the motor or pointer is caused to be held against the stop.
Preferably, the first predetermined radial distance is between 180xc2x0 and 360xc2x0, more preferably between 240xc2x0 and 350xc2x0.
Preferably, the second predetermined radial distance is between 1xc2x0 and 90xc2x0, more preferably about 10xc2x0.
In one form of the invention, a two stage calibration is provided whereby the instrument is calibrated on initial connection to an electrical power source as described above, and then undergoes a second calibration when electrical circuits with which the instrument is associated are switched on. With this arrangement, the second calibration comprises the steps of:
d. connecting the instrument to at least one electrical circuit for which the instrument is to be associated,
e. driving the motor at a reduced torque for a third predetermined radial distance whereby the motor or pointer contacts the stop, and
f. driving the motor at a reduced torque for said second predetermined radial distance to cause the motor or pointer to be held against the stop.
Preferably, the third predetermined radial distance is between 10xc2x0 and 120xc2x0, more preferably about 90xc2x0.
According to another aspect of the invention there is provided a system for calibrating an instrument or equipment operated by an electric motor, such as a stepper motor, comprising:
a pointer means engageable with said electric motor and adapted to be driven thereby,
a stop means positioned at a desired reference point wherein,
in a first powered state, the motor is accelerated at a maximum torque for a first predetermined radial distance to move said pointer towards a position whereat said stop is engaged,
thereafter, said motor is re-accelerated at a reduced torque for a second predetermined radial distance whereby the pointer is caused to be moved and held against said stop.
In one form of the invention, a two stage calibration is provided whereby, after an initial calibration, which occurs as soon as the instrument is first connected to a power source, it then undergoes a second calibration when electrical circuits with which the instrument is associated are switched on. In accordance with this aspect of the invention, the system includes a second calibration comprising,
in a second powered state in which the instrument is connected to at least one electrical circuit for which the instrument is to be associated, means for accelerating the motor at a reduced torque for a third predetermined radial distance whereby the motor or pointer driven thereby contacts the stop, and thereafter driving the motor at a reduced torque for said second predetermined radial distance to cause the motor or pointer to be held against the stop.
Preferably, the third predetermined radial distance is between 10xc2x0 and 120xc2x0, more preferably about 90xc2x0.
By calibrating the motor in this way, the need for a feedback control is avoided and calibration is achieved inexpensively, quickly and without jitter in both powered states.
In a preferred form of the invention, the electric motor is a stepper motor as used in an automotive instrumentation, where for correct operation of the instrument, calibration of the stepper motor is required.
The first powered state is preferably when an automotive instrument is first connected to a power supply, such as a battery. The second powered state is preferably when an ignition circuit of a vehicle is switched on.
The calibration is preferably executed in the first powered state whenever a battery power connection is interrupted and then re-connected. The calibration is preferably executed in the second powered state every time the ignition is switched on.
Further, in the first powered state, at maximum torque, a maximum motor rotation speed maybe of the order of 440xc2x0/s and said first predetermined radial distance may be 350xc2x0.
At the reduced torque, the motor rotation speed is preferably up to a maximum of 165xc2x0/s and said second predetermined radial distance may be 10xc2x0.
In the second powered state, at the said reduced torque, the motor rotation speed is preferably up to a maximum of 165xc2x0/s and said third predetermined radial distance maybe 90xc2x0.
In the first and second powered states, the motor or the pointer preferably contacts the stop when the motor is first rotated and completed at the radial distance of up to 350xc2x0 or 90xc2x0. The pointer may bounce off the stop upon completion of the rotation.
In practice, the pointer may be driven by an intermediate gear, contained within the motor housing which, in turn, is driven by a rotor of the motor. The pointer preferably contacts the stop and remains contacted or stationary against the stop once the motor has completed its second rotation at reduced torque, for 10xc2x0 and/or when the reference position is reached.
The pointer means may further comprise a pointer gear and the stop may comprise a mechanical stop. The stop may be located within the motor housing and contacted by the rotor of the motor, or by a co-operating part on the motor shaft, output gear or even an intermediate gear. For simplicity of description, however, reference will be made herein to the pointer contacting a stop, but it will be understood that this reference is not limiting.
The processing means preferably comprises a microprocessor which processes and controls the motor operation during calibration and normal operation.
It will be understood that during calibration of the stepper motor, the rotation of the rotor of the motor is in the opposite direction to normal instrument operation.
In order that the invention is more readily understood, a preferred embodiment will now be described with reference to the drawings.